14.3.4.2 The Match-one-or-more Operator (+ or \+)

If the syntax bit RE_LIMITED_OPS is set, then Regex doesn’t recognize
this operator. Otherwise, if the syntax bit RE_BK_PLUS_QM isn’t
set, then ‘+’ represents this operator; if it is, then ‘\+’
does.

This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression at least once;
see Match-zero-or-more Operator, for what it operates on, how some
syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that ‘+’ represents the match-one-or-more
operator; then ‘ca+r’ matches, e.g., ‘car’ and
‘caaaar’, but not ‘cr’.